A semiconductor device, such as a thyristor or silicon controlled rectifier, may be cooled by a pair of heat pipe structures that are fixed to the opposed major surfaces of the wafer, which forms the semiconductor portion of the device. A semiconductor device of this type is described in detail in U.S. Pat. No. 3,739,235 issued to Sebastian William Kessler, Jr. on June 12, 1973. The semiconductor wafer normally has a conductive cathode electrode coating on one major surface of the wafer and a conductive anode electrode coating on the opposite major surface of the wafer. Surrounding the cathode electrode coating on the same major surface of the wafer and electrically insulated from it, there is normally an annular conductive electrode coating forming the gate electrode of the device. One heat pipe structure is bonded to the cathode electrode coating while the second heat pipe structure is bonded to the anode electrode coating on the opposite wafer surface. Heat developed in the wafer during the operation of the device flows from the wafer into the two respective heat pipe structures where it is dissipated.
For many applications of thyristors, the current rating of the device is its surge current capability rather than its continuous current rating. The reason for this is that many uses of these devices are for motors and contactor, where a high short-circuit current occurs. The lower the temperature of the semiconductor wafer of these devices during a surge of current, the greater is the surge current capability of the device. It has been found that a semiconductor device, such as a transcalent thyristor, which is cooled with heat pipes has a higher continuous current rating per unit area of the emitter than when the device is cooled by other means. However, the surge capability of the device is only slightly better than these similar devices that are cooled by other means.
An analysis of the operation of semiconductor transcalent thyristors shows that the cooling of the semiconductor device by the heat pipe structures does not begin until 5 to 6 milliseconds after the current starts to flow. Any cooling during the first 5 to 6 milliseconds is due to the volume heat capacity of the materials adjacent to the major surfaces of the semiconductor wafer. In these semiconductor devices, the porous wick structure of the heat pipes is in direct contact with the cathode and anode electrode coatings respectively. This wick structure has 54% of the density of solid copper metal and is filled with water. Although the water has a large specific heat, it contributes little to the heat capacity of the wick because its density and its thermal conductivity are small. The poor thermal conductivity of the water limits the rate with which it is able to absorb the heat. For these reasons then, the heat pipes bonded to the semiconductor wafer lag in their heat dissipation after the current flow has started during the initial current surge.
When current initially begins to flow through the device, during the first 1/2 cycle of a 60 Hz current, for example, the peak surge of current can be one of 10,000 amperes at 5 volts, or with 50 kilowatts of power. If the heat generated is not instantly absorbed or dissipated, the device can be destroyed. This dissipation of heat must occur during the first 1/4 to 1/2 cycle of operation.